Conventionally, an electronic watch such as a wristwatch includes a stepper motor for driving an indicating hand, and such a hand, e.g., the second hand or the like, is moved based on a drive pulse generated by a reference signal from an oscillation circuit using a crystal oscillator or the like. Note that in the following, members to be driven by the stepper motor in the electronic watch, such as the indicating hand, are collectively referred to as indicators.
In recent years, many such kinds of electronic watch have been commercially available, which use a combination of a power generation system such as a solar battery and a rechargeable secondary battery such as a lithium-ion battery and are therefore free from battery disposal and battery replacement.
Those electronic watches have limited types of mountable power source in order to realize the reduction in size and thickness. It is therefore necessary to reduce power consumption of the stepper motor and the like as much as possible in order to perform a stable operation for a long time.
In light of this, there has been conventionally proposed an electronic watch including control means for reducing power consumption by recovering part of electric power used for driving the stepper motor (see, for example, Patent Literature 1). Hereinafter, an electronic watch including conventional power recovery control means is described with reference to the drawings.
FIG. 8(a) is a block diagram illustrating a configuration of the electronic watch including the conventional power recovery control means disclosed in Patent Literature 1. Reference symbol 11a denotes a motor control circuit including an oscillation circuit 111, a clock division circuit 112, and a drive pulse output circuit 113. In the motor control circuit 11a, the frequency of a reference signal generated by the oscillation circuit 111 is divided by the clock division circuit 112 to generate various timing signals, and the drive pulse output circuit 113 outputs a drive pulse S1 in response to the timing signals. Reference numeral 3 denotes a motor driver, which receives the drive pulse S1 as its input and outputs a drive pulse S2. Reference numeral 4 denotes a stepper motor for moving an indicating hand 5 based on the drive pulse S2. Reference symbol la denotes a clock circuit formed of the motor control circuit 11a and the motor driver 3. The clock circuit 1a obtains operating energy from a power source 2 formed of a secondary battery such as a lithium-ion battery.
FIG. 8(b) is an equivalent circuit diagram illustrating a configuration of the motor driver 3. Reference symbol 21p denotes a PMOS transistor having a gate to which a control signal φp1 is input, a source connected to the positive side of the power source 2, and a drain side connected to an OUT1 terminal. Reference symbol 21n denotes an NMOS transistor having a gate to which a control signal φn1 is input, a source connected to the negative side of the power source 2, and a drain side connected to the OUT1 terminal. Parasitic diodes 23p and 23n, which are generated because of the MOS transistor structure, are connected between the respective sources and drains. Reference symbol 22p denotes a PMOS transistor having a gate to which a control signal φp2 is input, a source connected to the positive side of the power source 2, and a drain side connected to an OUT2 terminal. Reference symbol 22n denotes an NMOS transistor having a gate to which a control signal φn2 is input, a source connected to the negative side of the power source 2, and a drain side connected to the OUT2 terminal. Parasitic diodes 24p and 24n, which are generated because of the MOS transistor structure, are connected between the respective sources and drains. A coil 25 and a series resistance component 26 of the coil 25, which constitute the stepper motor 4, are connected in series between the OUT1 terminal and the OUT2 terminal. Note that the control signals φp1, φn1, φp2, and φn2 to be input to the respective gates of the MOS transistors 21p, 21n, 22p, and 22n are signals constituting the drive pulse S1.
FIG. 8(c) shows a timing chart of waveforms of the control signals φp1, φn1, φp2, and φn2, waveforms of signals at the OUT1 terminal and the OUT2 terminal, and a waveform of a current flowing through the coil 25. Hereinafter, the operation of the motor driver 3 of the electronic watch including the conventional power recovery control means illustrated in FIG. 8(b) is described with reference to the timing chart. Normally (before time t0), the control signals φp1, φn1, φp2, and φn2 are held to Low level, and hence the PMOS transistors 21p and 22p are turned ON and the NMOS transistors 21n and 22n are turned OFF. Accordingly, the OUT1 terminal and the OUT2 terminal both have the same potential GND(+), and hence no current flows through the coil 25.
At time t0, the control signals φp1 and φn1 become High level, and hence the PMOS transistor 21p is turned OFF and the NMOS transistor 21n is turned ON. Accordingly, the OUT1 terminal becomes Low level and the OUT2 terminal becomes High level, and hence a current flows through the coil 25. That is, a rotor (not shown) constituting the stepper motor 4 rotates based on a magnetic field generated by the coil 25. When the control signals φp1 and φn1 become Low level at time t1, the OUT1 terminal and the OUT2 terminal both have the same potential GND(+), and hence the current supply to the coil 25 is interrupted. However, an induced current is generated because the rotor rotates by inertia. The rotor rotates toward a predetermined stop position, and then a magnetic flux passing through the coil 25, which is generated by the rotor, changes because of free oscillation of the rotor. The direction of the induced current flowing through the coil 25 also changes in accordance with the direction of the change of the magnetic flux.
At time t2, the control signals φp1 and φp2 are set to High level. The PMOS transistors 21p and 22p are turned OFF, and hence no induced current flows through the coil 25. Due to this abrupt change in current, a large counter-electromotive force is generated in the coil 25. When the counter-electromotive voltage at this time becomes higher than the voltage of the power source 2, a current flows from the coil 25 to the power source 2 via the parasitic diodes 23p, 23n, 24p, and 24n provided so as to connect the coil 25 and the power source 2 to each other. In this way, the power source 2 is charged.
Next, at time t3, the control signals φp1 and φn1 become Low level. The PMOS transistors 21p and 22p are turned ON, and hence an induced current flows through the coil 25. Due to this abrupt change in current, a counter-electromotive force having a polarity opposite to that of the counter-electromotive force at time t2 is generated in the coil 25. When the counter-electromotive voltage at this time becomes higher than the voltage of the power source 2, the power source 2 is charged similarly to the case at time t2. Such operation is repeatedly performed, for example, until time t4 while the rotor is generating induced electric power in the coil 25. In this way, part of electric power used for driving the stepper motor 4 can be recovered.
At time t4, the control signals φp1, φn1, φp2, and φn2 become Low level, and hence the PMOS transistors 21p and 22p are turned ON and the NMOS transistors 21n and 22n are turned OFF. Accordingly, the OUT1 terminal and the OUT2 terminal both have the same potential GND(+) and the free oscillation of the rotor is stopped, and hence no current flows through the coil 25.
Next, at time t5, the control signals φp2 and φn2 become High level, and hence the PMOS transistor 22p and the NMOS transistor 21n are turned OFF and the PMOS transistor 21p and the NMOS transistor 22n are turned ON. Accordingly, the OUT1 terminal becomes High level and the OUT2 terminal becomes Low level, and hence a current flows through the coil 25 in the direction opposite to the case at time t0. After that, the power recovery operation is performed from time t6 similarly to the above.
By the way, in the stepper motor used for an electronic watch, rotation detection may be performed to detect whether or not the rotor has rotated normally. As described in Patent Literature 2, the rotation detection is performed in such a manner that a drive pulse being an output for rotating the rotor is output and then the current waveform of an induced current resulting from inertial rotation of the rotor is detected. When it is continuously detected for a given period that the rotor has rotated normally, the output of the drive pulse is decreased to reduce power consumption. When the rotor has not rotated, a correction pulse for rotating the rotor is output to rotate the rotor reliably so as to prevent a delay of the electronic watch, and the output of the drive pulse is increased so that the rotor may rotate reliably in the next and subsequent operations.
On this occasion, the output level of the drive pulse is expressed by a ratio of the period during which the drive pulse is actually output to the period during which the drive pulse can be output, and is called duty ratio. In the case where the above-mentioned control is performed in an electronic watch, the lowest duty ratio needed to rotate the rotor normally is automatically selected and output.